‫ە‬Research Trends and Results

Field Research to Evaluate the Environmentof Rivers Downstream from Dams
AMANO Kunihiko, HeadHARANO Takashi, Senior Researcher
ENDO Maremi, Researcher
ITO Kanako, Researcher
River Environment Division, Environment Department
1. Introduction
Little past progress has been made in the evaluationof the impact of dams on the environments of therivers downstream from the dams, because the impact
manifestation mechanisms are complex and theimpacts themselves are not easy to abstract.
This research was undertaken to evaluate the impactof a dam on the riverbed of the river downstream fromthe dam. The impact of a dam on the riverbed
downstream from the dam is assumed to be a result of
the dam varying the downstream river’s flow regime
and the change of the flux of materials flowing
downstream, typified by the flux of sediment transport,
so dam quantities related to these were collected andevaluated at 79 dams nationwide. The riverbed
materials (soil forming the riverbed) and the benthic
organisms which inhabit this material were selected as
the indices used to evaluate the manifest impacts, anda field survey of these was carried out. 
2. Survey of dams nationwide and theirsurrounding rivers
In order to evaluate the quantities of sedimenttransport at 79 dams selected from all dams managed
by the national government throughout Japan, theannual average specific sedimentation was calculatedby dividing the annual average dam sedimentation byits drainage basin area, and at the same time, toevaluate the degree that dams change flow regimes,
the annual average maximum cut rate was calculatedby dividing the annual maximum difference between
the dam inflow and discharge by the annual maximum
inflow, and using these two indices, the dams wereclassified (Fig. 1; dams not suitable for the purpose,
those constructed separately upstream for example,were omitted.)
In rivers downstream from dams with large specificsedimentation (Group 2), the supply of sediment ought
to be sharply reduced. And downstream from dams
with small annual average maximum cut rate (Group 1and 3), the sediment flushing force is not reduced verymuch. Therefore, it is hypothesized that in rivers
downstream from dams in these groups (on the left
side of Fig. 1), the riverbed material will be coarser
grained or the riverbed will be degraded, and that

inversely, in downstream rivers in Group 4 where theflood cut is large, according to circumstances, theriverbed material can be fine-grained.
㻝㻜Group 1: flood cut – medium➨㻝䜾䝹䞊䝥䠖ὥỈ䜹䝑䝖䞉୰

Average 10-year
specific sedimentation (1,000m3/km2/y)
㻝㻜䜹ᖺᖹᆒẚሁ◁㔞㻔༓㼙㻟㻛㼗㼙㻞㻛㼥㻕Research Trends and Results(Key words) Dam, environment, field survey

Group 2: sedimentation – large➨㻞䜾䝹䞊䝥䠖ሁ◁䞉኱Group 3: flood cut – small➨㻟䜾䝹䞊䝥䠖ὥỈ䜹䝑䝖䞉ᑠ
Group 4. flood cut – large➨㻠䜾䝹䞊䝥䠖ὥỈ䜹䝑䝖䞉኱

㻝

㻜㻚㻝

㻜㻚㻜㻝
㻙㻜㻚㻞㻌㻜㻚㻜㻌㻜㻚㻞㻌

㻜㻚㻠㻌

㻜㻚㻢㻌

㻜㻚㻤㻌


Figure 1. Classification Based on Specific
Sedimentation Rate and Peak Cut
Averageannual maximum cut rateᖹᆒᖺ᭱኱䜹䝑䝖⋡

㻝㻚㻜㻌



A field survey was carried out longitudinally on
rivers upstream and downstream from dams. A
comparison of the 60% grain size of the riverbed
material and the shear stress at the riverbed calculated
based on the flow regime and shape of the river
channel revealed that in all groups, the 60% grain size
was relatively larger just downstream than it was in
the river upstream from the dams. This result suggeststhat just downstream from many dams, the possibilityof the riverbed becoming coarse-grained is high. 
The examination of results of the survey of benthic
organisms has confirmed that in addition to the damsin Groups 1 and 3, where it is assumed there is astrong tendency for the riverbed material of thedownstream river to be coarse-grained material, atdams in Group 2, downstream from the dams, species
which prefer a stable environment with riverbed
material somewhat larger than that upstream areincreasing.
3. Summary
In the future, we plan to interpret the data obtainedby a quantitative method such as hydraulic analysis to
study revealed patterns of the environment impact onrivers downstream from dams according to theproperties of the dams and countermeasure methods. 